This invention relates to the cutting of materials using spark erosion techniques.
In prior art spark erosion cutting of conducting materials, one or more cutting elements, all at a high positive or negative voltage with respect to the material being machined (which is sometimes earthed) are used such that sparks are generated between the cutting elements and the material. The spark causes degradation of the material, which erodes in a controlled manner, thus resulting in the desired cutting.
However, the above method of cutting cannot be used for insulating materials without first providing a conductive path between the region of the cut and the external circuit. In the case of diamond, it is necessary first to carbonize the surface of the diamond, so that a conducting path between the cutting element at high voltage and earth is present. Sparking between the cutting element and the diamond can then take place since it has been found that, on spark erosion, a layer of carbon is formed over the spark eroded regions, thus maintaining the conducting path. In the case of those insulators, for which no such conducting layer is formed during spark erosion or provided by other means, the prior art techniques will not cut the insulating material.
According to the invention, a method of cutting a material includes the steps of placing a pair of parallel spaced electrodes close to or in contact with the material, applying a voltage across the electrodes to produce a spark between the electrodes, and causing relative movement between the electrodes and material as the spark erodes the material to effect the cut. Preferably the erosion takes place in a suitable dielectric medium. To effect cutting movement of the electrodes through the material in the direction of cut must take place as the material is eroded. This movement may be achieved by moving the electrodes or the material or both.
The electrodes may be provided by the edges of a pair of parallel plates, which edges are preferably sharpened. Sharpened edges have the advantage that the spark will preferentially be produced at these edges. The plates may be of any suitable configuration. For example, the plates may be rectangular or they may be cylindrical, the one cylinder being inside the other. The electrodes may also be point electrodes.
In a preferred form of the invention, the electrodes are provided by a pair of parallel wires. The wires may lie in the same plane as the direction of relative movement of electrodes and material during cutting. Alternatively, the wires may lie in a plane which is perpendicular or at any other angle to this direction of movement.
The cross-sectional dimensions of the electrodes and the gap between the electrodes will vary according to the nature of the material being cut and the size of cut desired. For example, in the case of small objects such as diamonds the electrodes will be thin and the gap between them will be small. However, in the case of massive objects such as a block of metal or concrete, narrow cuts will generally not be required with a result that both the electrodes could be thick and the gap between the electrodes could be relatively large. Clearly the gap should not be so large that sparking is inhibited.
The voltage which is applied across the electrodes must be such as to produce a spark. The voltage applied will generally be between 500 and 20,000 volts. The rate of erosion and hence the cutting speed is dependent on the power dissipated by the spark. The power can be varied by adjusting the energy of the voltage pulse and the frequency of the pulse. However faster degradation leads to poorer surface finish. The parameters may be selected to achieve a desired surface finish.
As is mentioned above, the spark which is generated between the electrodes causes erosion for degradation of the material. As the material is removed, the electrodes and the workpiece are moved closer to each other, as discussed above, and the desired cut is produced. The electrodes may be moved through the material in the direction of cut by applying a load to the electrodes. The load forces the electrodes through the material as erosion takes place. The load which will be applied to the electrodes will vary according to parameters such as frictional force and mechanical tensioning of the electrodes. By way of example, it has been found that for wires and diamond, a load of between 1 to 50 grams applied to the wires produces a satisfactory cut. Alternatively, the electrodes may be kept sationary and the workpiece moved across the stationary electrodes by use of a suitable servo-mechanism as described fully in Electro-Erosion Machining of Metals by Livshits A. L., Butterworths, London 1960.
The method of the invention has particular application to the cutting of insulating materials due to the fact that it is not necessary by this method to form a conducting layer on the material. Examples of insulating materials which can be cut by the method of the invention are diamond, cubic boron nitride and ceramics such as aluminium oxide, spinel and porcelain. The invention also finds application with conducting materials such as tungsten carbide, cemented tungsten carbide, steels and other metals, diamond and cubic boron nitride compacts and semi-conductors such as silicon and germanium.
During cutting the workpiece is electrically floating and assumes a potential which depends on the particular operating conditions.
The spark erosion of the material may take place in a suitable dielectric liquid which serves to confine the spark to the cutting zone while at the same time acting as a coolant and a flushing agent for clearing particles detached from electrodes and the workpiece. Suitable dielectrics are liquids such as petroleum, kerosene, tetrachloromethane, trichloroethylene, tetraline, olive oil and paraffin oil. Generally these are liquids with high flash point which are non-conducting until the field breakdown strength is exceeded and which then deionize rapidly. A stream of the liquid can be poured over the zone in which sparking takes place. Alternatively, this zone may be immersed in a bath of the liquid.
In one particular example of the invention, the method utilises a pair of spaced parallel wires to cut diamond. The shape of the wires is not critical although for convenience the wires will generally have a circular cross-section. The wire thickness will in general be in the range 0.01 mm to 0.1 mm and the gap between the wires will generally not exceed 2 mm. The gap preferably lies in the range 0.025 to 2mm.
According to another aspect of the invention, there is provided apparatus for carrying out the above method including a pair of spaced parallel electrodes adapted to be placed close to or in contact with the material, means to apply a voltage across the electrodes to produce a spark in a zone between the electrodes, and means to move the electrodes and the material relatively to each other as the spark erodes the material to effect the cut. Each electrode preferably comprises a wire. Where the electrodes are provided by wires it is preferable to provide a feed roller and a take-up roller for the wires; the rollers being adapted continuously to feed wire through the spark zone. Furthermore, the apparatus may include means to vary the spacing between the wires in the spark zone. These means may comprise two spaced prongs, each prong straddling the wires and being rotatable about an axis which is transverse to the plane of the wires.